Pep Guardiola says he is disappointed Phil Foden, John Stones and Ilkay Gundogan are not part of Manchester City’s pre-season tour of the USA, admitting their absence contributes to “incredible problems” for his team. City are currently in Houston, where they are preparing to take on Club America on Thursday before travelling to Green Bay for a fixture with Bayern Munich on Sunday. But England internationals Foden and Stones, plus Germany international Gundogan, will miss the friendlies after failing to meet US entry requirements. Most travellers to the US must be fully vaccinated, which also prevented N’Golo Kante and Ruben Loftus-Cheek from taking part in Chelsea’s pre-season trip to the…